CARDIUM. 31 



mainder gradually becoming obsolete, or wearing off as age 

 advances. The ventral edge, which ascends most remark- 

 ably in front, displays but little convexity behind, scarcely 

 rising from the central arch on that side of the shell. The 

 front dorsal edge is straight or subretuse, and slopes most 

 rapidly, forming- a distant subcentral angle with the up- 

 turned ventral. The hinder dorsal edge, which is also 

 straight, becoming a little rounded at the extremity, in- 

 stead of declining, rises a little. The posterior margin is 

 almost straight, and when young is comparatively perpen- 

 dicular, becoming more oblique with age. The front ex- 

 tremity is very narrow, the hinder is very broad and bian- 

 gulated. The umbones are very prominent ; the beaks are 

 much inflected, and lie at about one-third the distance 

 from the anterior end ; in front of them is a rather large 

 flattened lunule-like space, which is free from costa? : the 

 ligament is almost imbedded. The internal surface is 

 whitish, with usually chocolate stains on the hinge-margin, 

 and a tinge of brown upon the hinder termination. The 

 lateral teeth are large, the front are the more approximate. 

 A large specimen before us has attained the amplitude of 

 five-eighths of an inch in length, and nearly half an inch in 

 breadth. 



The animal partakes of the triangular form of the shell, 

 and is much truncated posteriorly. The mantle is white, 

 and plain in front, fringed with numerous white filaments 

 about the bases of the siphonal orifices. The branchial 

 aperture is larger than the anal, and surrounded by a bor- 

 der of about ten filaments, as many or more tawny streaks 

 radiate from it, and there is a tinge of orange or tawny 

 around the other orifice also. Both are usually nearly 

 sessile. The foot is long, cylindrical, and large in propor- 



